I'm not an expert in the legal stuff, but GPL works can potentially be sold commercially - the source code would have to still be included, and although it isn't intended for that sort of thing, people have indeed sold for profit discs with GPL works without violation of the license.
As far as modified works, I don't know for sure about that part, I think the GPL code is supposed to be freely open so anyone can alter it and make their own version and pass it around, although I don't know exactly how that would or wouldn't work with media content included. That part might or might not work, I'm not sure on that point.
Sorry, I'm not trying to be difficult, but if there is an incompatible license it could still cause issues releasing it with the game.
Edit: Of course there is also more than one GPL license, they have different intentions, there's also other related licenses such as LGPL, etc...
Part of the problem is that all the licenses out there weren't really designed to all be compatible with each other, and also were not meant for the same type of material. (GPL is really meant more for programming code/computer software, CC licenses are more geared for music, media, creative works etc.) They have restrictions in different areas, and sometimes the only way to put them together is to release all components in the most liberal versions possible to avoid conflicts. It's complex and isn't always easy.